1. Field of the Invention
The present invention relates to a highly efficient piezoelectric element driving circuit that can vary driving voltage and consumes low power and a pump device that transfers a fluid by the vibration of a piezoelectric element.
2. Description of the Related Art
A piezoelectric element makes a mechanical movement according to an applied voltage because a piezoelectric material is bent when an electric field is applied to the piezoelectric material. Utilizing this characteristic of converting electric energy into kinetic energy, piezoelectric elements are widely applied to actuators, motors, and the like.
A driving circuit in which a resonant element such as a capacitor or the like forming a resonant circuit is connected to a primary side coil of a transformer, and primary side driving frequency is made variable is known as a driving circuit for a piezoelectric element of an oscillatory wave motor (see Japanese Patent Publication No. Hei 5-16277 (hereinafter referred to as Patent Document 1), for example).
A new actuator using a mode of coupled-vibration with a peripheral structure rather than a discrete piezoelectric element has recently been proposed. In such an actuator, a piezoelectric element and the peripheral structure vibrate in a mechanically coupled manner. Thus, behavior in the coupled-vibration mode at this time is complex. For example, rather than one electric resonance point, there are a plurality of frequency resonance points that are not in a simple high-order relation. In such an actuator, the Q-value of mechanical resonance frequency often becomes high. When driving frequency differs from mechanical resonance frequency of the actuator by 10%, for example, there occurs a phenomenon in which an amount of mechanical output produced by the actuator is decreased significantly.
In a case where the driving circuit described in the above-mentioned Patent Document 1, for example, is applied to such an actuator, when the driving frequency is changed for a purpose of changing mechanical output over a wide range, the driving frequency becomes different from the mechanical resonance frequency of the actuator, and thus mechanical output from the actuator becomes substantially zero.
It is necessary to use a driving circuit that controls the mechanical output of the actuator while operating at a frequency coinciding with the mechanical resonance frequency.
A method of combining a variable output voltage power supply with a driving circuit is conceivable for a purpose of controlling the mechanical output of the actuator at a fixed driving frequency. This method is also described in Patent Document 1. However, a variable output voltage power supply circuit on a relatively large circuit scale is necessary separately from a driving circuit.
A driving circuit is known in which a reactance element such as an inductor or the like forming a parallel resonant circuit together with a load (a piezoelectric element or a piezoelectric motor) on a secondary side of a transformer is provided, and driving frequency is stabilized by optimizing a constant of the reactance element so as to make the resonance frequency of the parallel resonant circuit coincide with the driving frequency (see Japanese Patent No. 2976489 (hereinafter referred to as Patent Document 2), for example).
In the driving circuit described in the above-mentioned Patent Document 2, because the driving frequency is determined by the parallel resonant circuit on the secondary side of the transformer, a switch circuit on a primary side has only a function of regularly performing boosting operation for secondary side voltage and supplying energy. Patent Document 2 discloses a switch circuit that applies a power supply voltage to a midpoint of a primary winding of the transformer and that alternately grounds both ends of the primary winding in order to realize a zero cross waveform of the secondary side voltage.
In the driving circuit disclosed in Patent Document 2, there is a need for suppressing a harmonic component due to a ripple and reducing a loss by making a boosted voltage generated on the secondary side of the transformer a regular sine wave.
Therefore, in the driving circuit described in the above Patent Document 2, power supply of the primary side of the transformer is performed via an inductance element, and the inductance element and a parallel capacitance in an equivalent circuit of an ultrasonic motor as a load are made to perform parallel resonance so that the parallel capacitance is apparently cancelled.
The driving circuit having such a configuration and operation can remove a ripple and reduce a loss, and also makes it possible to set the driving duty ratio of a switching element connected to the primary side of the transformer at a simple 50%.